Geomorphic Response to Catastrophic Flooding on the Umatilla River, Oregon.
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Date
2025-02-24
Authors
Merrill, Nicole
Journal Title
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Publisher
University of Oregon
Abstract
Using aerial imagery and digitization, this thesis studied changes to geomorphology and vegetation on the Umatilla River after catastrophic flooding in 2020 and 1996-7, compared geomorphic effectiveness of the two floods, and tested potential control variables. On average, the channel widened, shifted laterally, built bars, and stripped vegetation, though changes were slightly less pronounced in 1996-7 than in 2020. Internal thresholds that maintain channel width and sinuosity as well as the lower discharge in 1996 contributed to this difference. Active channel widening and lateral movement were the most sensitive to the control variables in 2020 and in 1996-7. Channel sinuosity played the largest part in controlling shear stress and, therefore, planimetric changes. However, bar accretion, which was influenced the most by sediment availability, was more depended on tributary input. Comparison of lateral activity and channel widening in 1996-7 and 2020 showed the downstream reaches that had not been channelized underwent the most change from both floods, which reflects previous flood effects on this river. Climate change and uncertain future flood risk make understanding river response to flooding extremely important for river planners managing community safety and ecological restoration.